Silver ions are widely used in industrial and antimicrobial applications, but their high mobility and toxicity can create serious risks when they enter aquatic environments. A new study published in Biochar shows that nano-biochar can rapidly convert dissolved silver ions into silver nanoparticles under carefully controlled alkaline conditions, revealing a key role for superoxide radicals in the process.
"Our findings show that nano-biochar is not simply a passive adsorbent. Its surface chemistry can actively regulate electron transfer and reactive oxygen species production, allowing silver ions to be transformed in a highly pH-dependent way," said Wei Zhu, corresponding author of the study. "Understanding this mechanism could help guide the design of more efficient carbon-based materials for water remediation and metal recovery."
The research team produced two forms of nano-biochar by ball milling wood chip-derived biochar that had been pyrolyzed at either 400°C or 700°C. The resulting materials, called nano400 and nano700, were then tested for their ability to reduce silver ions, or Ag+, to silver nanoparticles, or AgNPs, across a range of pH conditions.
The results showed a clear chemical switch controlled by pH. No silver nanoparticles formed under acidic to neutral conditions between pH 6.5 and 7.5. In contrast, weakly alkaline conditions from pH 8.5 to 9.5 strongly promoted silver ion reduction, with the highest nanoparticle yield observed at pH 9.5.
Nano400 was especially effective. It contained more oxygen-rich surface groups, including phenolic hydroxyl groups, and had a stronger electron-donating capacity than nano700. At pH 9.5, nano400 produced silver nanoparticles at a reaction rate approximately 2.5 times higher than nano700. The process occurred within about 10 minutes at room temperature without external light or other energy input.
Mechanistic experiments revealed that superoxide radicals, a type of reactive oxygen species, were the critical intermediates driving the reaction. Under alkaline conditions, oxygen-containing groups on nano-biochar become deprotonated, increasing surface charge and helping silver ions bind to the material. Persistent free radicals on the nano-biochar surface then interact with dissolved oxygen to generate superoxide radicals, which reduce the surface-bound silver ions to metallic silver nanoparticles.
The researchers also found that more nano-biochar was not always better. Excessive dosages reduced nanoparticle formation, likely because high particle concentrations increased aggregation, altered the availability of reactive sites, and promoted self-consumption of reactive oxygen species.
The findings provide a more detailed picture of how nano-biochar structure, pH, and reactive oxygen species work together to control metal transformations in water. They also suggest that lower-temperature biochar, which retains more oxygen-containing functional groups, may offer advantages for designing reactive nano-biochar materials.
The authors note that the experiments were conducted in ultrapure water. Future work will need to evaluate performance in real wastewater, where dissolved oxygen, salts, competing ions, and other contaminants may influence the reaction. Long-term stability, material reuse, and the environmental safety of the resulting silver nanoparticles will also require further study.
Overall, the study provides a mechanistic foundation for developing nano-biochar-based approaches to silver-contaminated water treatment and potentially to the recovery of valuable metals from wastewater.
===
Journal Reference: Gu, S., Wang, D., Wang, T. et al. pH-regulated surface chemistry of nano-biochar for selective silver ion reduction: the pivotal role of superoxide radicals. Biochar 8, 133 (2026).
https://doi.org/10.1007/s42773-026-00651-7
===
About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
